In information recording media having a recording layer using the properties of magnetism, light, or magnetooptics, a typical one is a magnetic disc. Conventionally, aluminum substrates have been widely used as substrates for magnetic discs. However, in recent years, to meet the requirement for reduction in magnetic head levitation amount for increasing the recording density, more glass substrates are used because their surface is smoother than that of aluminum substrates and they has less surface defect.
In order to reduce surface defect of such glass substrate for information recording media, and thus to realize high-density recording, it is necessary to allow gas bubbles generated in a melting process of glass to remain in the glass substrate as few as possible, which gas bubbles have been produced in. Conventionally, there is typically used a method in which As2O or Sb2O3 is contained as a fining agent in the glass components to remove (fine) the gas bubbles in the molten glass (see Patent Document 1).
However, since As2O3 and Sb2O3 are toxic, use of these substances is becoming restricted in more occasions from the viewpoint of environment and human health. For this reason, studies have been done for a method for removing the gas bubbles in the molten glass without using As2O3 or Sb2O3 as a fining agent, and there is proposed a method in which the molten glass is decompressed to remove the gas bubbles (see Patent Document 2).